Carboxylic acid halides palladium complexes

The phase-transfer method has also been employed for the carbonylation of benzylic halides to carboxylic acids. The palladium(O) complexes [Pd(PPh3)4] (103), [Pd(diphos)2] (104) and [Pd(DBA)2] (105 DBA = dibenzylideneacetone) were used as catalysts. With (103) and (104) the carboxylic acid was the major product. Complex (105) gave little or none of the acid, the toluene and bibenzyl derivatives corresponding to the benzyl halide used being formed. Benzyl esters of the carboxylic acid were sometimes present as minor products. The reaction has been adapted to provide a new synthesis of anthranilic acid derivatives (equation 106). Tri-n-butylamine was used to neutralize the HBr formed. 

Palladium complexes also catalyze the carbonylation of halides. Aryl (see 13-13), vinylic, benzylic, and allylic halides (especially iodides) can be converted to carboxylic esters with CO, an alcohol or alkoxide, and a palladium complex. Similar reactivity was reported with vinyl triflates. Use of an amine instead of the alcohol or alkoxide leads to an amide. Reaction with an amine, AJBN, CO, and a tetraalkyltin catalyst also leads to an amide. Similar reaction with an alcohol, under Xe irradiation, leads to the ester. Benzylic and allylic halides were converted to carboxylic acids electrocatalytically, with CO and a cobalt imine complex. Vinylic halides were similarly converted with CO and nickel cyanide, under phase-transfer conditions. ... 

Complexes of other metals are also capable of catalyzing useful carbonylation reactions under phase transfer conditions. For example, certain palladium(o) catalysts, like Co2(C0)g, can catalyze the carbonylation of benzylic halides to carboxylic acids. When applied to vinylic dibromides, unsaturated diacids or diynes were obtained, using Pd(diphos)2[diphos l,2-bis(diphenylphosphino)ethane] as the metal catalyst, benzyltriethylammonium chloride as the phase transfer agent, and t-amyl alcohol or benzene as the organic phase(18),... 

Arene(tricarbonyl)chromium complexes, 19 Nickel boride, 197 to trans-alkenes Chromium(II) sulfate, 84 of anhydrides to lactones Tetrachlorotris[bis(l,4-diphenyl-phosphine)butane]diruthenium, 288 of aromatic rings Palladium catalysts, 230 Raney nickel, 265 Sodium borohydride-1,3-Dicyano-benzene, 279 of aryl halides to arenes Palladium on carbon, 230 of benzyl ethers to alcohols Palladium catalysts, 230 of carboxylic acids to aldehydes Vilsmeier reagent, 341 of epoxides to alcohols Samarium(II) iodide, 270 Sodium hydride-Sodium /-amyloxide-Nickel(II) chloride, 281 Sodium hydride-Sodium /-amyloxide-Zinc chloride, 281 of esters to alcohols Sodium borohydride, 278 of imines and related compounds Arene(tricarbonyl)chromium complexes, 19... 

Carboxylic acid chlorides and chloroformate esters add to tetrakis(triphenylphosphine)palladium(0) to form acylpalladium derivatives (equation 42).102 On heating, the acylpalladium complexes can lose carbon monoxide (reversibly). Attempts to employ acid halides in vinylic acylations, therefore, often result in obtaining decarbonylated products (see below). However, there are some exceptions. Acylation may occur when the alkenes are highly reactive and/or in cases where the acylpalladium complexes are resistant to decarbonylation and in situations where intramolecular reactions can form five-membered rings. 

The reaction sequence in the vinylation of aromatic halides and vinyl halides, i.e. the Heck reaction, is oxidative addition of the alkyl halide to a zerovalent palladium complex, then insertion of an alkene and completed by /3-hydride elimination and HX elimination. Initially though, C-H activation of a C-H alkene bond had also been taken into consideration. Although the Heck reaction reduces the formation of salt by-products by half compared with cross-coupling reactions, salts are still formed in stoichiometric amounts. Further reduction of salt production by a proper choice of aryl precursors has been reported (Chapter III.2.1) [1]. In these examples aromatic carboxylic anhydrides were used instead of halides and the co-produced acid can be recycled and one molecule of carbon monoxide is sacrificed. Catalytic activation of aromatic C-H bonds and subsequent insertion of alkenes leads to new C-C bond formation without production of halide salt byproducts, as shown in Scheme 1. When the hydroarylation reaction is performed with alkynes one obtains arylalkenes, the products of the Heck reaction, which now are synthesized without the co-production of salts. No reoxidation of the metal is required, because palladium(II) is regenerated. 

Aryl, vinyl, and benzyl halides are catalytically carbonylated by CO generated in situ from chloroform and aqueous alkali, in the presence of phosphine-palladium complexes such as PdCl2(PPh3)3. The biphasic reactions do not require a PT agent, they occur at r.t., and they afford the corresponding carboxylic acids in up to 92% yield. Labeling experiments with CHCb showed that the source of the Cl unit in the carbonylation is chloroform... 

The hydrophilic palladium complex (2) was also a good catalyst for the carboxylation of benzyl halides under heptane-water two-phase conditions [20]. Benzyl chloride and bromide give phenylacetic acid in high yields under mild conditions (Eq. 5). However, the biphasic carboxylation with PdCI2(PPH,) , is very slow, and gives a considerable amount of benzyl alcohol. The addition of a normal PTC such... 

The carbonylation of chloroarenes has been described by Alper and Grushin [27] and Jenner and Bentaleb [28], While the former showed that square-planar complexes of divalent palladium, [ L2PdCl2], where L = tertiary phosphine, are active catalysts for the biphasic carbonylation of aromatic halides, including chloroarenes (when L = tricyclohexylphosphine), to the corresponding carboxylic acids, the latter demonstrated that chloroarenes can be converted into aromatic acids via catalytic reaction with aqueous methyl formate under biphasic conditions. [ PdCl2(PCy3)2] was the most efficient catalyst. The addition of [ Ru3(CO)12] and ammonium formate improved yield and selectivity of the carbonylation reaction. The mechanism should involve oxidative addition of the C—Cl bond to a zero-valent Pd species followed by CO insertion. However, the palladium catalyst may also directly activate methyl formate. Compared to other carbonylations of aryl-Hal compounds the procedure is quite convenient (no solvent, no initial pressurization) [27]. 

Other metal catalysts which have been utilized for biphasic carbonylation of ben-zylic halides to carboxylic acids under phase-transfer conditions, besides cobalt carbonyl [11], include palladium(O) complexes [12] and water-soluble nickel cyanide complexes [13], Although not investigated in detail, it must be assumed that catalysis takes place in all these reactions in the organic phase. 

Cuprous chloride tends to form water-soluble complexes with lower olefins and acts as an IPTC catalyst, e.g., in the two-phase hydrolysis of alkyl chlorides to alcohols with sodium carboxylate solution [10,151] and in the Prins reactions between 1-alkenes and aqueous formaldehyde in the presence of HCl to form 1,3-glycols [10]. Similarly, water-soluble rhodium-based catalysts (4-diphenylphosphinobenzoic acid and tri-Cs-io-alkylmethylam-monium chlorides) were used as IPTC catalysts for the hydroformylation of hexene, dodecene, and hexadecene to produce aldehydes for the fine chemicals market [152]. Palladium diphenyl(potassium sulfonatobenzyl)phosphine and its oxide complexes catalyzed the IPTC dehalogenation reactions of allyl and benzyl halides [153]. Allylic substrates such as cinnamyl ethyl carbonate and nucleophiles such as ethyl acetoactate and acetyl acetone catalyzed by a water-soluble bis(dibenzylideneacetone)palladium or palladium complex of sulfonated triphenylphosphine gave regio- and stereo-specific alkylation products in quantitative yields [154]. Ito et al. used a self-assembled nanocage as an IPTC catalyst for the Wacker oxidation of styrene catalyzed by (en)Pd(N03) [155]. 

In parallel with progress on the oxidation of alkanes to alcohols and alkyl halides have been reports on the oxidative carbonylations of alkanes and arenes to form carboxylic acids. Fujiwara showed that stoichiometric amounts of arylpalladium acetates formed from the reaction of Pd(OAc)j with arenes and that the resulting arylpalladium complex reacts with CO in acetic acid to form aromatic acids (Equation 18.22). When O, BuOOH, alkyl halides, or K SPj were added as oxidant, the reaction became catalytic in palladium, and benzoic acids were generated from benzene, CO, and the oxidant in the presence of palladium acetate (Equation 18.23). Tl-ie highest yields were obtained with KjSPg as oxidant. Sen has reported related oxidation reactions in acidic media. - ... 

The electrosynthesis of aromatic carboxylic acids from aryl halides and carbon dioxide is either catalyzed by nickel " or palladium complexes. 

Before discussing the double carbonylation processes it may be helpful to understand the mechanism of the single carbonylation of aryl halides into carboxylic acid derivatives (Heck processes). The first step in the catalytic process is oxidative addition of an aryl halide to Pd(0) species formed from a catalyst precursor to yield an arylpal-ladium halide intermediate (A) in Scheme 1. Insertion of carbon monoxide into the aryl-palladium bond in A gives an acylpalladium halide complex (B). Attack of a nucleophile such as alcohol, amine, and water assisted by a base on the acylpalladium complex yields carboxylic ester, amide, and carboxylic acid, although details of the mechanism have not been unequivocally established. The palladium(O) species regenerated in the process further undergoes oxidative addition to carry out the catalytic cycle (Scheme 1). 

Scheme 3.16) [30], This catalytic system mediates the extrusion of COj from aromatic carboxylates to generate arylcopper species, and the palladium complex catalyzes the cross-coupling of these intermediates with aryl halides, allowing the direct coupling of several aryl, heteroaryl, or vinyl carboxylic acids with aryl or heteroaryl iodides, bromides, or chlorides at 160 °C in the presence of KjCOj. 

In this process, carboxylic acids and olefins are converted to vinylarenes in the presence of palladium in catalytic amounts and silver in overstoichiomettic amounts (Scheme 4). It is initiated by formation of a silver arenecarboxylate salt, which then reacts with a palladium(ll) halide complex (a) to give the silver halide and palladium carboxylate b. The extrusion of carbon dioxide, and thus the conversimi of the carboxylate to the organometallic species c, occurs within the ligand sphere of palladium. The subsequent steps, i.e., insertion of the olefin into the palladium-carbon bond, internal rotation, and (3-hydride elimination, correspond to the classical Heck reaction [26]. An additional oxidation step converts the palladium(0) species f back to palladium(ll). It is this oxidation by silver that... 

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